The importance of a serine at the inner mouth of the channel (end of domain IV S6 segment) was studied. Its mutation to alanine abrogated the ability of the channel to enter into mode 2 gating upon stimulation by activation of PKA or addition of BAY-K8644. The serine (Ser 1517) may be a key regulatory site that determines, in coordination with the previously studied Ser 1142 in the selectivity filter, the responsivenes of the channel to phosphorylation and the dihydropyridine agonist. On the other hand, mutation of a Glycine to to Arginine in the S6 helix of domain I, creates a consensus phophorylation site for Ser 439 in Timothy's syndrome patients and has the opposite effect of mutating Ser 1142 to Ala. This may be the explanation for the excitotoxisity in Timothy's syndrome patients. Mode 2 gating may also be the basis for excitotoxicities associated with chronic cyclosporin treatment, which would be prmoting mode 2 gating by preventing de-phophorylation at Ser 1142.